Stationary mounted film projectors in cinemas are always located at a predetermined distance from the projection screen in the theater. Focusing the projected image, i.e. adjsuting the optics of the projector, should, in theory, be a non-recurring task, but experience has shown that in practice re-focusing has to be carried out from time to time. For example, deposits from the surface of the film can build up in the film path of the projector which results directly in blurred images, and furthermore the layer structure of the film can vary.
The development within the cinema business has lead to that an operator nowadays has to supervise 8-10 projectors in different auditoria. Quite naturally he is not able then to pay much time and attention to the focusing of the various projectors, which could be inconvenient to the audience, since sharpness is a determining factor when it comes to impression and experience of the picture shown. Also, it can be difficult to find swiftly the correct setting of focus and, in addition, in certain picture sequences it can be hard to judge the proper sharpness of the picture from the projection room. It is often so that the projectionist focuses with the guidance of the translated subtitles occurring in foreign film along the lower edge of the picture. This means that sharpness becomes poorer where best needed, namely, within that part of the screen where the actor's faces mostly appear, i.e. the central area of the screen, at a distance from the top edge of the screen corresponding to about one third of the height of the screen.
In practice there is thus a need for simple and correct focusing means, and earlier several solutions to the problem of focusing have bene proposed. For example, in the Swiss Patent Specification No. 484 443 an apparatus for focusing projected images is described. The basis of this solution is the fact that a sharp image exhibits greater contrasts between light and dark portions, and that a blurred image includes grey tones, i.e. the contrast effect is eliminated. However, it is difficult in practice, in particular when cinematographic pictures are concerned, to utilize grey tone conditions for providing focusing parameters. Several specific steps have to be taken in order that normal fluctuations in image intensity be eliminated and in order that the two measuring signals be obtained which are to form the basis of a correcting signal.